Avoidance of over-tightening screws during fracture plate fixation or lag screw insertion is a goal in orthopaedics, as well as orthopaedic resident training In orthopaedics, screws are often inadvertently stripped when they are used to affix a fracture fixation plate to a broken bone. Screw stripping during internal fixation of displaced lateral malleolar fractures, for example, occurs in up to 88% of patients more than 50-years-old, whereas inadvertent and unrecognized screw stripping may occur at a rate of 20% in cortical bone. Determining an appropriate endpoint for screw insertion has been an elusive task. Much effort has been focused on identifying a torque limit for screw insertion. However, torque is a function of screw pitch, bone density and thickness, and bone-thread interfacial friction.
Torque is converted by means of the screw threads into screw tension, which provides the compressive force on the fixation plate to maintain fracture reduction. Experienced orthopaedic surgeons typically tighten screws to 86% of maximum torque clinically, which is presumably “two-fingers tight”. Inserting screws beyond 70% of the maximum torque, at least in ovine tibiae, compromises pullout strength. Because peak torque depends on the material and geometric properties of bone, it is virtually impossible clinically, a priori, to know the peak torque for screw insertion in a given bone at a given location to stay below the 70% peak value. Once a screw is stripped, its pullout strength is reduced by more than 80% and may lead to loss of fracture reduction and implant failure.
An alternative to the torque-limiting method of tightening fasteners (screws) is the “turn-of-the-nut” method that uses a rotational limit and is the preferred method used in building construction. The method works by placing sample bolts in a proofing machine. The nut is tightened until snug and then rotated until the desired tension (specified by design) in the bolt is reached. The amount of rotation needed to achieve the design tension in the bolt, and thereby compression across the joint, is then applied to all nuts and bolts of the type proofed. In engineering applications, the nut and bolt are of the same material, and the bolt size is selected by design so that the load of a given bolt is well below its failure load.
In orthopaedic application, the bone assumes the role of the nut, and the screw size is dictated by the geometry of the bone, fracture and plate to be used. Because the failure properties of bone are much less than those of the screws, failure likely occurs in the threads cut into the bone. We assume that failure in bone threads is related to the rotation of the screw. Thus, once a screw is inserted so that the head is snug against the plate, there is a degree of rotation that will achieve optimal fixation without causing stripping and loss of screw purchase. However, many inexperienced surgeons cannot adequately determine when the head of the screw is seated against the plate or bone. The surgeon is also left to guess how much further to rotate the screw for optimal fixation.
Accordingly, there is a need in the art for an instrumented screw insertion tool that guides the surgeon to achieve optimal fixation.